1. Field of the Invention
The present general inventive concept relates to a method and apparatus to determine an encoding mode of an audio signal and a method and apparatus to encode and/or decode an audio signal using the encoding mode determination method and apparatus, and more particularly, to an encoding mode determination method and apparatus which can be used in an encoding apparatus to determine an encoding mode of an audio signal according to a domain and a coding method that are suitable for encoding the audio signal.
2. Description of the Related Art
Audio signals can be classified as various types, such as speech signals, music signals, or mixtures of speech signals and music signals, according to their characteristics, and different coding methods or compression methods are applied to the various types of the audio signal.
The compression methods for audio signals can be divided into an audio codec and a speech codec. The audio codec, such as Advanced Audio Coding Plus (aacPlus), is intended to compress music signals. The audio codec compresses a music signal in a frequency domain using a psychoacoustic model. However, when a speech signal is compressed using the audio codec, sound quality degrades, and the sound quality degradation becomes more serious when the speech signal includes an attack signal. The speech codec, such as Adaptive Multi Rate-WideBand (AMR-WB), is intended to compress speech signals. The speech codec compresses an audio signal in a time domain using an utterance model. However, when an audio signal is compressed using the speech codec, sound quality degrades.
In order to efficiently perform speech/music compression at the same time based on the above-described characteristics, AMR−WB+(3GPP TS 26.290) has been suggested. AMR−WB+ is a speech compression method using algebraic code excited linear prediction (ACELP) for speech compression and transform coded excitation (TCX) for audio compression.
AMR−WB+ determines whether to apply ACELP or TCX for each frame on a time axis. Although AMR−WB+ works efficiently for a compression object that approximates a speech signal, it may cause degradation in sound quality or compression rate for a compression object that approximates a music signal. Thus, when different compression methods are applied according to the characteristics or modes of an audio signal, a method for determining an encoding mode has a great influence on the performance of encoding or compression with respect to the audio signal.
U.S. Pat. No. 6,134,518 discloses a conventional method for coding a digital audio signal using a CELP coder and a transform coder. Referring to FIG. 1, a classifier 20 measures autocorrelation of an input audio signal 10 to select one of a CELP coder 30 and a transform coder 40 based on the measurement of the autocorrelation. The input audio signal 10 is coded by one of the CELP coder 30 and the transform coder 40 selected by switching of a switch 50. The conventional method selects the best encoding mode by the classifier 20 that calculates a probability that the current mode is a speech signal or a music signal using autocorrelation in the time domain.
However, because of weak noise tolerance, the conventional method has a low hit rate of mode determination and signal classification under noisy conditions. That is, the mode determination and signal classification are inaccurately performed. Moreover, frequent mode oscillation in frame units cannot provide a smooth reconstructed audio signal.